The present disclosure relates to a signal quality evaluation apparatus and a signal quality evaluation method that are suitable for performing a decoding process using partial response maximum likelihood (PRML) on a reproduced signal from a recording medium, for example, and relates to a reproducing device that is provided with the signal quality evaluation apparatus and that performs reproduction of information.
For example, regarding an optical disk serving as an example of the recording medium, compact discs (CDs), digital versatile discs (DVDs), and Blu-ray Discs (BDs (registered trademark)) are widely used setting forth the history of high densification of recording.
In particular, in the case of high density recording at the level of a BD, a method using a technique called a partial response maximum likelihood (PRML) detection is typically used as a bit detection method.
PRML is a technology that combines a process of partial response and a technology of maximum likelihood detection. Partial response refers to a process of returning an output longer than one bit in response to a one-bit input, that is, a process of making a determination by a plurality of input bits of the output. In particular, a process of obtaining a reproduced signal as a signal obtained by multiplying an input of four consecutive information bits by 1, 2, 2, and 1 in this order and adding the results, as often used for optical disks such as the Blu-ray Disc and the like, is expressed as PR(1, 2, 2, 1).
Maximum likelihood detection is a method of defining a distance referred to as a path metric between two signal strings, determining a distance between an actual signal and a signal predicted from an assumed bit sequence, and detecting a bit sequence providing the closest distance. Incidentally, the path metric is defined as a distance obtained by adding the squares of differences in amplitude between two signals at same times over a whole time. Viterbi detection is used to search for the bit sequence providing the closest distance.
Partial response maximum likelihood combining these methods is a method of adjusting a signal obtained from bit information on a recording medium such that the signal is in a partial response process by a filter referred to as an equalizer, determining a path metric between the resulting reproduced signal and the partial response of an assumed bit sequence, and detecting a bit sequence providing the closest distance.
Based on the principle of PRML detection, a method using a metric difference (also referred to as a SAM value) distribution that indicates the margin of a Viterbi detector for selecting a path is already typically used as a method for evaluating the reproduced signal quality of an optical disk.
For example, a signal quality evaluation method has been disclosed in each of the Patent Literatures 1, 2, 3, and 4 described above that has a satisfactory correlation with error rates of PRML even when high-density recording is performed on a conventional optical disk.
In each of the methods, in a PRML class that is actually used, an index value is formed by extracting several error patterns that statistically have high frequency of error occurrence, by obtaining an index value of each of the error patterns, and by integrating the obtained index values together.
Since the error patterns each have a different metric difference distribution (a mean value of the distribution and a variance), the error patterns cannot be dealt as a single distribution and, thus, the above is performed.